Quadrupole mass spectrometers have proven to be useful as general purpose mass analyzers. These devices are four rod structures and when designed for operation in a resolving mode are usually about 20 cm in length and require extreme mechanical precision in terms of fabrication and alignment. When operated in a resolving mode quadrupole mass spectrometers conventionally have both RF and DC voltages applied to them. Values of these voltages vary with the frequency and mass range of operation, but can be on the order of 1600 volts (peak-to-peak) RF for operation at 1 MHz and .+-.272 volts DC for a rod array inscribed radius r.sub.0 of 0.418 cm and a mass range of 600 Daltons. The high degrees of mechanical and electrical sophistication required means that the costs of these mass spectrometers are high.
The most common mode of operation teaches that the operating line q should cross just below the tip of the first stability region. The stability region is plotted as a function of the well-known Mathieu parameters a and q. Operation at the tip of the first stability region means that only ions with a narrow range of m/z will be transmitted, giving the potential for high mass resolution.
In theory operation with no applied DC voltage equates to operation along the horizontal axis of the Mathieu stability diagram and this should give transmission of a wide range of ions up to an m/z given by the limit of the first stability region.
However, as taught in U.S. Pat. No. 4,090,075, a quadrupole rod array can provide mass resolution in the absence of applied resolving DC voltages. This so called RF-only mode of operation has several advantages over conventional RF/DC operational modes. Conventional RF/DC quadrupole rod mass spectrometers supply mass resolution based on the intrinsic stability or instability of given ions within the rod structure in the combination of the time varying RF and the time independent DC fields. In contrast to the more common RF/DC quadrupole mass analyzers, mass resolution for an RF-only instrument is thought to occur when ions that are only marginally stable with a particular applied RF voltage gain excess axial kinetic energy in the exit fringing field of the rod structure. Since a large part of the phenomena leading to mass resolution of an RF-only mass analyzer occurs at the exit of the rod array the length limitations characteristic of RF/DC resolving quadrupoles no longer apply and mechanical tolerances for rod roundness and straightness are considerably relaxed. Finally, there is no need for a high precision high voltage DC power supply in the RF-only mode of operation. Taken together the inherent advantages of RF-only operation suggest the opportunity for a much smaller and less costly mass analyzer than conventional RF/DC quadrupoles. Although the potential of such a device is significant problems such as sample dependent background from high velocity ions and clusters have considerably limited the commercial use of RF-only mass analyzers, especially when coupled with electrospray and other atmospheric pressure ion sources.
In the RF-only mode, to separate out ions with higher energies, energy filtering is accomplished, typically by placement of a retarding grid either at the exit of the quadrupole or further downstream. This has the effect of separating particles having higher energy, i.e. those ions with a q near 0.907 which have acquired a higher kinetic energy, from other ions with lower kinetic energy.
A drawback associated with this energy filtering technique is that there can be a significant high energy tail in the energy distribution of ions entering the quadrupole rods, i.e. ions with a q substantially less than 0.907. These high energy ions can originate from a variety of sources, but the net effect is overlap of the energy distribution of these ions, and of the curve representing ions with a q of near 0.907. This in turn results in the appearance of a continuum background signal upon which overlaps the peaks of the ions with a q near 0.907.
One technique proposed for reducing this background is in U.S. Pat. No. 4,189,640. This teaches providing a centrally located attractively biased disk of appropriate size, located after the analyzing quadrupole. The intention is to reduce high velocity and higher mass species. However, in practice this also reduces analyte ion intensity off-setting much of the expected gains in signal-to-noise ratio.